Coder for code modulation transmission



NOV. 9, 1948. c, NORWINE 2,453,454

CODER FOR CODE MODULATION TRANSMISSION Filed Aug. 51, 1946 INVENTOR A.C. NORW/NE A T TORNEL Patented Nov. 9, 1948 CODER FOR CODE MODULATIONTRANSMISSION Andrew C. Norwine, Short Hills. N. J., assignor to BellTelephone Laboratories. Incorporated, New York, N. Y., a corporation ofNew York Application August 31, 1946, Serial No. 694,256

7 Claims.

This invention relates to coders for use in code modulation transmissionsystems of the type disclosed in a oopencling application for a UnitedStates patent, Serial No. 677.667 of J. C. Schelling filed June 19,1946, and more particularly to improved apparatus for expressing theinstantaneous amplitude of a complex wave to be transmitted as a binarycode group of voltages suitable for transmission to a remote point.

In code modulation transmission systems the instantaneous amplitude ofthe complex wave to be transmitted is converted into a code group ofsignaling; voltages representative of that amplitude. In such systemsthe code may be based upon the binary counting system in accordance withwhich each of the plurality of signaling voltages may represent one ofthe denominational orders of the binary number and thus represent afixed portion of the total possible amplitude of the complex waveexpressed as a binary numbe Each of the signaling voltages has twovalues and in one value represents the presence of fixed portion. of thetotal possible amplitude while in the other value it represents theabsence of that portion. In general. the fixed portions of the total arerelated in amplitude as powers of no. Since each of the signalingvoltages may have either of two values, the total number of permutationswhich can be obtained between 11 signaling voltages is 2 Thus a threeelement code permits unique expression of 2 or eight discrete amplitudesand similarly a five element code permits expression of 2 or thirty-twodifferent amplitudes.

One type of apparatus for generating such binary code groups ofsignaling voltages involves a coder for providing separate signalingvoltages individual. to each of the denominational orders of the binarynumber code and suitable for transmission cver separate channels to aremote station. Since such coders must provide separate signalingvoltages, each of which is referred to round potential to facilitateindividual transmission. in separate channels they have necessarily beenrelatively complex employing numerous vacuum tubes and associatedcircuits. Such complex increase the space, power and maintenancerequirements of the equipment and limit the usefulness of this type ofequipment in many applications.

It is an object of the invention to provide means for continuouslyexpressing the instantaneous 0.rnplitude of a complex wave in terms of aplurality of bi-valued signals each representing a 2 fixed portion ofthe total possible amplitude of the complex wave.

It is a further object of the present invention to provide a coder ofthe type referred to above which will be small, light in weight, havelow power requirements and avoid the relatively complex circuitarrangements common to previous coders of this general type.

The invention relates in one aspect to a coder comprising a plurality ofrelays or vacuum tubes each controlling an output signal representativeof a difierent fixed portion of the total possible amplitude of thecomplex wave to be transmitted,

means for applying the complex wave to all of the relays in parallel andmeans for applying individually to each of the relays separate voltagesderived from all of the relays which control signals representing largerportions of the total possible amplitude, such voltages beingrespectively proportional to the output signals of the last-mentionedrelays.

- The coder of the invention employs one relay or vacuum tube for eachdenominational order of the binary code. Thus, if an n--element code is"employed, the coder of the present invention comprises n vacuum tubes orrelays, one for each element of the code, plus a single input tubecommon to all of the coding elements. of illustration a coder embodyingthe present invention is arranged to express the instantaneous amplitudeof the complex wave by a three-ele-' or positive or negative signalsapproximately four units in amplitude.

The above and other features of the invention will be described in thefollowing detailed specification and pointed out in the accompanyingclaims.

In the drawing, the single figure is a schematic diagram of a codingcircuit in accordance with the invention.

' As shown in the drawing, the coder forexpress ing the instantaneousamplitude of a complex wave in three-element binary code comprises se-"lector tubes or relays l0, l2 and I4 controlling separate output signalswhich appear at terminals lii; l6 and 20-, respectively. Inaccordancewith For purposes m i" the binary coding system considered above, eachof these selectors controls a voltage represents tive of a fixedportionof the total possible amplitude of a complex wave to be transmitted,these portions being related in amplitude as powers of two. Thusselector ill controls an output representing one unit of amplitude .ofthe complex wave while selectors l2 and IQ control outputs representingrespectively two and four units. By suitable control means, theoperation of these selectors may be switched so that the combination oftheir output signals forms any one o'f eig'ht different permutations.Conveniently, the uriit of measurement employed may be one volt and inthis case eight different amplitudes from O to? volts inclusive may beexpressed uniquely.

Conveniently, each of selectors it, 12 and it may comprise a triode typevacuum tube although other types of vacuum tube circuits having twodifferent operating conditions or conditions of stability may beemployed with equally advantageous results. The plates of selector tubesill, l zand id are connected through load resistors 22, 24cm 26,'rfiSDGCtiVely, to the positive terminal of battery 28, the negativeterminal of which is connected .to ground. In addition the plates ofthese mo or-sa e respectively connected to out: put terminals l6, l8 and2t]. 'lhecathodes of the selector tubes are connected directly to groundwhile their 'grids are connected respectively throughrcsistors' .30, 32and 34- to .the negative terminal of bias battery fili'the positive,terminal of which is connected to ground In addition to the aboveconnections the plate of .selectortube i2 is connected through resistor:15

3d tol'the' grid of selector tube it) while the plate of selector. tubeM is connected throughresistor 41) to .thegrid of selector tube 12 andthrough rese ter; tothe grid of selector tube ill.

The complex wave to be represented in binary code form 'for transmissionis applied atter ninal L4 and thence throu h a blocking capacitor it toid'of alcathodefollower .thegrid circuit of w 11 includes resistor 46 inseries with bi'asbat: tel- 1; 'llhis cathode follower may convenientlycd nprise a 'triode' type Vacuum tube theplate of w h is connected tothe positive terminal or a ry' .50 the negative terminal of which isgrounded, the cathode of thejtube being. connected through loadresistor52 to ground. ihe voltage appearingacross load resistor 52 isan, accurate'r'e; pro notion of the voltageapplied at terminal ill andis applied through resistors 54!. .555 and 58; respectively in parallelto the control grids of selectors i0, i2 and L4,

Inrlmnsidering'the operation of the coder of the invention, let it,besassumed that the biasvoltage from battery M is positive and'substantiallyequal tonne-half the largest possible peak to pealramplitude of thecomplex wave tobe transmitted. Insurer words, the biasvoltage is such'lthat' in the absence of applied complex wavedor whenthe complex .Wave is of zero amplitude) thcvoltage at the output'ofcathod'e'follovver d8 will fall at the approximate center of theoperating range of the coder. Then all of the voltages appearing acrosscathode resistor 52 (and applied through series resistors h 6 and-583ccthe selectors .will be of positive polarity and will tend to oppose thenegatiye-biasapplied. to these selectors from battery 36.

Inbrder to simplify the following description of operational thecoderIit is convenientto measure the amplitudes of the voltagesappearing across cathoderesistor 52 of cathode rouo erts inrespect toground. It will .bere'rnembered "that the temples wave has zeroamplitude, and

ill

. plitude of between three and f reaches itsmaximum possible posit).lngt'he-iollowing description all input vo t units) when amplitude ofseven units when the complex wave amplitude. ges to exemplary coderstotal range oi oi the coder will be those appearing acro. fiflmeasuredin respect to ground cor pm the summation of the complex wave a'op atterminal y! with the positive bias contri on from battery 41.

The negative bias from battery ef fective in the absence or voltagesacross cathode resistor 52hr cathode follower E8 to out off the flow ofcurrent in the selector tubes. Accordingly, the plate voltages of thesetubes rise and relatively high 'voltages app'ear at output terminals i6,i8 and 25!.

Because of the interconnections between selectors I2 andifl, betweenselectors M and i2 between selectors M and l El, positive voltages areapplied to the grid of selector ill whenever selectors {2 or M are cutoff and to the grid of r ctor whenever selector l l is out on. F1 7more, whenever selectors I? or M become satur ted due to the applicationof a sufficiently high positive ge totheirgrids, the positive voltagetrib tions therefrom to the grids of other select'o'rs are removed.

' In the grid circuit of selector l2, resistors 5B and 32ersso'proportioned that at the grid the positive voltage contribution fromcoder l l when out off bears the same relation to the portion of thetotal amplitude represented by coder H!- in this example, 4 units) asthe input voltage contribution at the grid applied through resistor Eiils to the amplitude of the complex applied at input terminal 44. In otherwords, the voltage contribution at the grid of selector'lii correspondtoan input signal of amplitude A has an amplitude of a arbitrary units,then the positive contribution from selector i when cut oil has anamplitude of 4 in the same arbitrary ibution I. when out off is ofamplitude 4 in the same arbitrary units and that from selector i2 is ofamplitude 2 in the same arbitrary units. It should be noted that thearbitrary units employed measuring. the relative amplitudes of thevoltages applied to the several selectors needs not be the same for eachselector so long as the relationships defined .aboveare preserved in aconsistent system of units at the input or" each selectcr.

It will be recalled that in the absence of other applied voltages, thenegative bias from batry is effective tocut off each of the This biasvoltage is also sufficient to r a item. lcctor. [2 cut oli despite thepositive contribution from cut-off selector i l and to maintain selectorHi cut off "despite the positive contributions from cut-bit. selectorsHand M. Since two such posi tive contributions are applied to selectorID a lower input signal amplitude is required to saturate this selectorthan is required to saturate selector [2 in which the input signal isaided by only one positive contribution.

More specifically, in the exemplary system considered herein, thenegative bias voltage and the proportions of the resistances in the gridnetworks of the selectors are so chosen that, when all selectors are cutoff an input signal of at least one volt is required to saturateselector Ill. Similarly, when selector I4 is cut oil an input of 2 voltsis required to saturate selector l2 and an input of 4 volts is requiredto saturate selector M. If the positive contributions from selector I4to selector l2 and from selectors l2 and Hi to selector ID are removed,the input voltage must be increased correspondingly to cause saturationof the selector involved.

Turning now to the detailed operation of the circuit, let it first beassumed that the input signal across resistor 52 is zero. Then each ofselectors I0, 12 and I4 is cut off and high voltages which may denotethe zeros of a binary number, appear at output terminals I6, 18 and 20.The binary code group obtained by setting down the values (one or zero)of the outputs at these terminals is thus 000. Hereinafter such binarycode groups will be written by listing the signals appearing atterminals 20, I8 and it in that order. This corresponds to listing thecode element representing the highest denominational order of the binarynumber first reading from left to right in the usual manner.

If, for example, the instantaneous amplitude of the complex wave to betransmitted is such that one volt appears across resistor 52, one voltis applied to each of resistors 54, 56 and 58. As to selector ill, thepositive voltage corresponding to the applied l-volt signal plus thepositive contributions from cut-off selectors I2 and i4 is suffi cientto overcome the negative bias from battery 36 and to cause the flow ofsaturation current through selector ID. The voltage applied to selectorl2 in response to the l-volt signal is in suflicient, even with thepositive contribution from selector l4, to overcome the negative [biasand this selector remains cut off. Similarly, the voltage applied toselector I4' in response to a l-volt signal is not sufficient toovercome the negative bias on this selector. Accordingly, with theapplication of a signal of one volt, selectors i2 and 14 remain cut offand selector lo becomes saturated. The voltage at output terminal l6drops to a relatively low value and the binary code group obtained bylisting the signals on the output terminals becomes 001.

Let it now be assumed that the complex wave amplitude is such that aninput signal of two volts appears across load resistor 52 of cathodefollower 48. In the absence of other considera tions, this voltage wouldbe sufficient to maintain selector ID at saturation. It would also causethe application of a positive voltage of sufiicient amplitude inaddition to the positive contribution from selector II to cause the flowof saturation current in selector l2. Resistors 58 and 34 associatedwith selector [4 are so proportioned that an applied voltage of twovolts is insufficient to overcome the negative bias applied to thisselector. It will he noted, however, that when selector 12 becomessaturated due to the application of two volts, its plate potential islowered and the positive potential applied to the control grid ofselector I0 from this tube is reduced. Accordingly, in'the' absence ofthis positive contribution, which is proportional to two volts asapplied to selector ID, the negative bias from battery 36 is againeiTective to out 01f selector it. Thus, for an input of two voltsselectors it) and Id are out ofi while selector i2 is saturated so thatthe binary code group obtained at output terminals as, it and i6 is 010corresponding to an amplitude of two.

If a signal of three volts is now applied to the selectors the followingaction occurs. Because of the relative proportions of resistors 58 and3t, selector l4 remains cut off, the applied voltage being insufficientto overcome the negative bias from battery 36. Selector 92 becomessaturated since an input signal of only two volts is required tosaturate this selector so long as selector 14 remains cut oil and thepositive contribution therefrom to selector i2 is maintained. On theother hand, the input of three volts produces a suffi-- ciently largepositive voltage at the grid of selector 10 to overcome the negativebias from battery 36 despite the absence of the positive contribution(which is proportional to two volts) from selector i2. Accordingly,selectors iii and iii are saturated and the binary code group obtainedon the output terminals is 011 corresponding to an amplitude of 3.

Further, if the input voltage across resistor 52 rises to four voltsselector M is saturated. When this selector becomes saturated, however,its plate voltage drops and the positive contributions from thisselector through resistor til to the grid of selector l2 and throughresistor A 2 to the grid of selector It are removed. In the case ofselector l2 the removal of the positive contribution from selector i iseffective to balance the 4-volt input signal for this selector and topermit the negative bias from battery 38 to cut ofi the tube. When thisoccurs the plate voltage of selector l2 rises and a positivecontribution is again applied from selector !2 through resistor 38 tothe grid of selector Iii. It will be recalled, "however, that an inputsignal of one volt is required to saturate selector iii even whenpositive contributions from both selectors l2 and it (proportional totwo plus four or six volts as applied to selector i 0) are applied toaugment the signal.

It is thus apparent that the sum of the positive voltages applied toselector it must at least be proportional to seven volts to saturatethis selector. Here the sum of the positive voltages is proportional tosix volts, four from the input and two from selector i2, sot-hatselector is cut oil and the code group appearing at the output terminalsbecomes 100.

In a similar way the application of input signals greater than fourvolts causes appropriate changes in the conditions of selectors iii, i2and M. For example, upon the application of five volts selector l4remains saturated. This increased signal voltage, however, is suficientto make the sum of the positive voltages applied to selector l0proportional to seven volts, five from the input and two from selectorl2, and this tube is saturated. On the other hand, the increase of theapplied voltage is not suflicient to saturate selector 12 which remainscut off and the output code group is 3.0. The operation of the circuitin response to inputs of six and seven volts may be analyzed in anentirely analogous manner.

It will be understood that a wider range of amplitudes may be expressedin binary code group form by increasing the number of selectors. In eachcase connections must be made between each of the added selectors andthe grid'circuit's of all selectors representing smaller portions of thetotal possible amplitude of the complex wave to be transmitted. Suchconnections must be so made that the contribution from each selector toeach selector controlling a signal representative of a smaller portionof the total possible amplitude bears the same relation (in the selectorto which it is applied) to the portion of the total amplituderepresented by the originating selector as the signal voltage appliedto'the selector rethe contribution of does to the input signal.

The coding circuit of the invention may be used in the transmitters ofvarious types of code modulation transmission systems. For example, theseparate voltages at terminals 1 0, l2 and [4 of the illustrativeembodiment may be individually and simultaneously transmitted or eachoutput terminal may be sampled and the voltages thereon transmitted insequence.

What is claimed is:

1. In a communication system, a coder for generating representations ofthe instantaneous amplitude of a Wave to be transmitted according to acode of a plurality of code elements each representative of a differentfixed portion of the total possible amplitude of said Wave, said codercomprising a plurality of relays each controlling an output signalcorresponding to one of said code elements, means for applying said Waveto all of said relays, means for deriving from the outputs of all ofsaid relays except that representing the largest portion of the signalamplitude volta es proportional to the amplitude portions which therelays respectively represent, and means for applying the voltagederived from each relay to each other relay controlling a larger derivedvoltage.

2. In a communication system, a coder for generating representations ofthe instantaneous amplitude of a Wave to be transmitted according to acode of a plurality of code elements each representative of a differentfixed portion of the total possible amplitude of said wave, said codercomprising a plurality of relays each continuously controlling an outputsignal corresponding to one of said code elements, means forcontinuously applying said Wave to all of said relays, means forderiving from the outputs of all of said relays except thatrep-resenting the largest portion of the signal amplitude voltagesproportional to the amplitude portions Which the relays respectivelyrepresent, and means for applying the voltage derived from each relay toeach other relay controlling a larger derived voltage.

3. In a communication system, a coder for generating representations ofthe instantaneous amplitude of a Wave to be transmitted according to acode of a plurality of code elements each representative of a differentfixed portion of the total possible amplitude of said Wave, said codercomprising a plurality of electron tubes each controlling an outputsignal corresponding to one of said code elements, means for applyingsaid Wave to all of said electron tubes, means for deriving from theoutputs of all of said electron tubes except that representing thelargest portion of the signal amplitude voltages proportional to theamplitude portions which the electron tubes respectively represent and.means for applying the voltages derived from each electron tube to eachother electron tube controlling a larger derived voltage.

4. In a communication system, a coder for generating representations ofthe instantaneous amplitude of a Wave to be transmitted according to acode of a plurality of code elements each representative of a differentfixed portion of the total possible amplitude of said wave, said codercomprising a plurality of electron tubes each controlling an outputsignal corresponding to one of said code elements, means for applyingsaid wave to all of saidv electron tubes in parallel, means for derivingfrom the outputs of all of said elec-- tron tubes except thatrepresenting the largest portion of the signal amplitude voltagesproportional to the amplitude portions which the electron tubesrespectively represent, and means for applying the voltage derived fromeach electron tube in parallel to each other electron tube controlling alarger derived voltage.

5. In a communication system for transmitting a message Wave, apparatusfor generating code representations of the instantaneous amplitude ofthe message Wave to be transmitted comprising a plurality of electrontubes each controlling an output signal representative of a differentfixed portion of the total possible amplitude of said message Wave,means for applying to each of said electron tubes voltages proportionalto the instantaneous amplitude of the message Wave, and means forapplying individually to each of said electron tubes separate voltagesfrom all electron tubes controlling signals representing larger portionsof said total amplitude, said voltages as applied being proportional tothe parts of the total amplitude represented by the electron tubes fromwhich they are respectively derived in the same units as those in whichthe voltage proportional to the complex Wave is measured.

6. In a communication system for transmitting a message wave, a coderfor generating code representations of the instantaneous amplitude ofthe message wave, said coder comprising a plurality of selectors eachcontrolling an output signal representative of a diiierent fixed portionof the total possible amplitude of said message wave, means for applyinga fixed bias voltage to each of said selectors, means for applying avoltage proportional to the instantaneous amplitude of the message Wavein parallel to each of said selectors and in opposition to said biasvoltages, and means for applying to each selector except thatcontrolling the output signal representing the largest portion of thetotal possible amplitude additional control voltages, these voltages ineach case also opposing said bias voltages and being respectivelyproportional to the amplitudes represented by the output signals ofselectors controlling signals representative of larger portions of saidtotal amplitude.

7. In a communication system for transmitting an intelligence Wave, acoder for generating code representations of the instantaneous amplitudeof an intelligence wave to be transmitted, said coder comprising aplurality of electron tubes each controlling an output signalrepresentative of a different fixed portion of the total possibleamplitude of said intelligence wave, means for applying a fixed biasvoltage to each of said electron tubes, means for applying to each ofsaid electron tubes and in opposition to said bias voltages voltagesproportional to the instantaneous amplitude of said intelligence wave,and means for applying individually to each of said electron tubesseparate additional voltages from all electron tubes controlling signalsrepresentative of larger portions of said 9 10 total amplitude, thesevoltages also opposing said bias voltages and, as applied to eachelectron REFERENCES CITED t b being proportional t th parts of th t t lThe following references are of record in the amplitude represented bythe electron tubes file of this P t t! from which they are derived inthe same units 5 UNITED STATES PATENTS as employed in measuring theinstantaneous amplitude of the intelligence wave applied to the re- NumbName Date spective tubes. 2,272,070 Reeves Feb. 3, 1942 2,282,046Goldsmith May 5, 1942 ANDREW C. NORWINE. 9 Valensi Mar. 9, 1943

